In a vapor compression refrigeration system, low pressure gas refrigerant is compressed to a high pressure gas, which is then condensed in a condenser to a liquid. The liquid refrigerant is evaporated in an evaporator into the low pressure gas. Several systems have been proposed which utilize the energy of the liquid refrigerant that flows from the high pressure side (the condenser side) to the low pressure side (the evaporator side) to improve the overall efficiency of the refrigeration system. For example, U.S. Pat. Nos. 3,934,424, 4,170,116, 4,086,772 and 4,208,885 teach the use of an expansion engine in the vapor cycle to improve the overall efficiency of the refrigeration system. However, these and other systems have gained very little or no commercial acceptance. Furthermore, the art, in general, has taught against the use of expansion engines in refrigeration systems. For example, David Mooney in the textbook, Mechanical Engineering Thermodynamics states on page 467, line 2, "In actual cases, after allowing for the irreversibility of the real engine process, the gain by use of the expansion engine is usually negligible and such machines are not used in modern vapor refrigeration plants."
In a closed loop refrigeration system, potential energy is stored in the refrigerant pressure difference between the high side pressure and the low side pressure. Energy is wasted when this potential energy is changed into kinetic energy in the expansion valve of the refrigeration system. Also, energy is stored in the liquid refrigerant temperature on the high pressure side, which is changed into kinetic energy of the molecules when the liquid refrigerant boils in the evaporator. The prior art refrigeration systems attempt to improve the efficiency by utilizing this kinetic energy to drive or operate an expansion engine, which in turn is used to perform some useful function. However, the prior art systems do not overcome the energy waste because these systems do not properly control the expansion valve throttling process and the flow of the refrigerant through various elements of the refrigeration system.
The use of an expansion engine as taught in prior art systems, i.e., on an evaporator, produces an inherent conflict, which can be understood by considering the following two extremes of the flow of the refrigerant through the evaporator.
In the one extreme, if the refrigerant leaving the expansion engine is completely vaporized, there will be little refrigeration accomplished by the system.
In the other extreme, if liquid refrigerant is allowed to enter the compressors, unnecessary load will be placed on the expansion engine causing a loss of efficiency or mechanical failure.
The present invention provides a refrigeration system which uses a centralized expansion engine to improve the overall efficiency of the refrigeration system. The use of a centralized expansion engine avoids the above described inherent problem associated with the prior art systems that use expansion engines in the evaporators.
Further improvement in efficiency of the refrigeration system of the present invention is obtained by properly controlling the flow of the refrigerant through various elements of the refrigeration system and by subcooling the liquid refrigerant in the condenser at all ambient temperatures.
It has been known in the art that the net refrigerating effect in a refrigeration system can be improved by subcooling the liquid refrigerant before evaporating it in the evaporators. Subcooling the liquid means that some energy is taken out of the liquid refrigerant and, as a consequence, it does not have to be removed by the expansion process in the cooling evaporator, thus, improving the overall efficiency of the refrigeration system. One method of subcooling the refrigerant has been to accumulate the liquid refrigerant leaving the condenser in a reservoir and then circulating that liquid refrigerant through another cooling section. This method produces subcooling at a small additional operating cost, but at an increased capital cost because such a system requires a second condenser and an additional amount of the refrigerant. The use of additional refrigerant is undesirable for the reasons given below.
Also, to achieve subcooling, it is typical to equip the condenser with a flood control device which elevates the condensing pressure of the refrigeration system during low ambient temperatures by reducing the effective condenser surface which is available for condensing. This is accomplished by partially filling (flooding) the condenser with the liquid refrigerant when the condensing pressure is not sufficient. These systems also require greater amounts of refrigerant to accomplish flooding as the ambient temperature drops. The use of additional refrigerant is undesirable because commonly used refrigerants contain Chloro-Fluoro-Carbons ("CFCs") which are harmful to the environment because when they leak into the atmosphere they contribute to the depletion of the earth's upper atmosphere. Refrigerant leaks occur several times over the life of most refrigerant systems. Consequently, the use of additional amounts of refrigerant may dramatically increase the amount of leakage of CFCs from the refrigeration systems into the atmosphere.
Refrigeration systems currently available also attempt to maximize the subcooling effect during the colder periods of the year, i.e., at lower ambient temperatures. One such system is described in U.S. Pat. No. 4,831,835, which performs subcooling during periods of low ambient temperature by utilizing a relatively complicated valve arrangement. This system, however, ignores the subcooling at some ambient temperatures.
U.S. Pat. No. 4,621,505 also discloses means for improving the subcooling effects during low ambient conditions. With respect to subcooling at higher ambients, this patent suggests that in summer operations when the ambient temperature is above 85 to 90 degrees Fahrenheit, the condensation temperature and head pressures will be higher and little or no economic benefit can be expected. The need to benefit from subcooling has been known for some time in the refrigeration industry; however, to date, no method for achieving subcooling in a condenser at all ambient temperatures (high or low) appears to have succeeded in the marketplace. It should be noted that subcooling of a refrigerant to a temperature that is closer to the ambient temperature of the refrigeration system will improve efficiency at all times of the year. Thus, subcooling within the condenser itself at all times, i.e., at all ambient temperatures, is a desirable feature to have in a refrigeration system.
Another type of a subooling system is disclosed in U.S. Pat. No. 4,136,528. It describes a system which provides subcooling to a degree that is sufficient to ensure that the expansion valves operate properly in colder ambient conditions. This system, too, fails to provide subcooling during summer to obtain energy savings.
The prior art systems which utilize subcooling in the condenser have failed to recognize the necessity of holding the refrigerant in the liquid state for some time before allowing it to leave the condenser. In order to make thermal expansion valves function, hold-back valves have been used in the condensate line leaving the condenser to elevate the condensing pressure during low ambient conditions. This method produces liquid subcooling when the condenser is flooded. The hold-back valves used for this purpose have throttling ranges from fully open to fully closed of 20 to 60 psi, which means that an additional inefficiency due to higher condensing pressures during higher ambient and higher flow conditions is introduced.
The refrigeration system of the present invention also addresses the above described subcooling problems associated with the prior art systems and provides a system and method for subcooling the liquid refrigerant in the condenser at all ambient temperatures while utilizing the least amount of refrigerant. Thus, the present invention provides a refrigeration system which utilizes a centralized expansion engine and effects subcooling of the refrigerant in the condenser at all ambient temperatures to improve the overall efficiency of a refrigeration system.